A polarizing film for use in an image display (particularly, liquid crystal display) is required to have both high transmittance and high degree of polarization in order to provide bright images with high color reproducibility. Such a polarizing film is conventionally produced by a process including allowing a polyvinyl alcohol film (hereinafter referred to as “PVA film”) to swell with swelling water and then adsorbing and orienting iodine having dichroism or a dichroic material such as a dichroic dye in a dyeing step, and performing uniaxial stretching (Patent Document 1 below).
In the general production method mentioned above, however, stretching is performed while immersion in a dyeing liquid is performed in the dyeing step, which may result in excessive adsorption of iodine or the dichroic dye on the PVA film. This may cause a problem in which the orientation of the adsorbed iodine or the like is degraded so that the resulting polarizing film can cause light leakage in the short wavelength region (380-410 nm) of visible light (380-780 nm).
The method described in Patent Document 2 below is a method for producing a polarizing film having a high degree of orientation and prevented from the light leakage. This production method includes subjecting a PVA film to dry stretching, then adsorbing and orienting iodine or a dichroic dye, and then stretching the film while immersing it in an aqueous boric acid solution. Examples of the dry stretching described include general methods such as a uniaxial stretching method using a heating roller and a roll-to-roll uniaxial stretching method in which stretching is performed between rolls placed in an heating oven, while a tensile force is applied.
In the above production method, however, there is some difficulty in adsorbing a sufficient amount of iodine or the like onto the PVA film, because the PVA film is heated, dry-stretched to a stretching ratio of about 4 to 5 times, and subjected to the dyeing step without being subjected to a swelling step. In the above production method, therefore, it is difficult to produce a polarizing film with a high degree of polarization. There is also a problem in which the PVA film may be ruptured when the PVA film is stretched to a high stretching ratio in the aqueous boric acid solution so that the polarizing properties can be improved.